The invention relates to a screw for fastening metal and/or plastic sections or plates to a substructure, comprising a shank with a boring portion and a threaded portion as well as a drive portion for applying a driving tool.
The very act of fastening sections or plates to a substructure is problematic in that such parts must achieve a condition in which they press firmly against the substructure when fastened. These negative effects are intensified when relatively stiff sections or plates become further distorted under certain circumstances or exhibit a longitudinal twist. In this case it is usually not possible to work with a depth limiter for the driving device. Even if torque clutches are used, it is still difficult to stop the process at the precise time, since an associated torque increase occurs as soon as the screw head bears on the section to be fastened, thus stopping the driving device. Further driving movement is then stopped regardless of whether or not the metal and/or plastic section or plate to be fastened continues to bear on the substructure.
The object of the present invention was therefore to provide a screw of the type mentioned in the introduction, in the use of which the section or plate to be fastened bears snugly on the substructure in the finally set condition.
According to the invention, this is achieved by the fact that the portion of the shank adjacent to the drive portion flares conically toward the drive portion, and that the diameter of the end of the conical portion of the shank close to the drive portion is larger than the diameter of the borehole receiving the screw in the section to be fastened and thus is also larger than the boring diameter of the boring portion.
By virtue of these features according to the invention, the metal and/or plastic section to be fastened is subjected to initial tension directed toward the substructure and thus is pressed thereagainst as soon as the screw has been driven in by one or more rotations. Because of the associated friction in the borehole and of the conical shape, the conical portion of the shank adjacent to the screw head exerts a force in the screwing direction on the section to be fastened.
It then becomes a simple matter to adjust an appropriate torque clutch such that it disengages only at appropriately high torque. By that time, however, the section to be fastened has already been pressed snugly against the substructure by the action of the conical portion, and so all that remains is for the driving tool to be stopped as soon as the appropriate torque is reached.
The action of the conical portion on the borehole wall also produces extremely good locking against reverse rotation. The surface of the conical portion is for practical purposes disposed in a snug-fitting seat in the borehole wall. Furthermore, in the particular case of portions that have become distorted or twisted in the process, said portions have a tendency to return to their initial condition. Thereby an additional resilient load is always exerted on the screw, intensifying the locking effect against reverse rotation, especially in view of the action of the conical portion.
It is intuitively obvious that the beginning of the conical portion must extend from the shank or from an unthreaded portion of the shank, because this conical portion, while following the shank, must be inserted easily into the borehole in the section to be fastened. Thereafter, however, it is necessary that the conical portion engage as rapidly and effectively as possible with the borehole wall, so that thereby it can apply the axial initial tension optimally. In this way the tightening torque and therefore the axial initial tension are exerted on the section to be fastened as soon as the screw has been driven in a relatively short distance, before the process of driving the screw is fully accomplished.
Another advantage of the conical portion is that optimum fastening to a substructure is possible even if said substructure comprises a thin metal sheet or other thin materials. Since the conical portion for practical purposes represents the agency which initiates an associated torque increase, stripping of the thread in the substructure cannot occur.
From DE-OS 2157212 there is already known a screw in which a conical shank portion is provided at the head end of the shank to form a thicker region in order thereby to provide the shank with reinforcement to compensate for a drive slot for a screwdriver. Furthermore, incisions are provided on the underside of the head of the known screw in order to cut a recess into the workpiece and in this way to countersink the screw head in the workpiece.
From DE-OS 2238191 there is also known a fastening means which always comprises a screw and nut. The screw is drawn into a borehole by tightening the nut, and therefore is not itself rotated. In this case the conical portion of the screw is dimensioned such that the frictional force developed by tightening the nut prevents rotation of the shank in the hole. Even if the driving tool is designed to stop at a specified torque, it could not be stopped by such friction.
In another screw known from DE 4019157 C1, a shaped portion is provided in the region of the screw head. This portion engages in the material of the substructure in order to cause a sudden large increase in driving torque of the screw immediately upon contact with the substructure. This effect is desired for screws to be driven into a substructure with poor pull-out resistance, such as gas-formed concrete. Such an effect is not desired in the use of a screw in the manner according to the invention, nor is a shaped portion of this type employed. Instead, the increasing friction between a conical portion and the borehole is utilized to stop a driving tool by torque-dependent effect.
In another proposal according to the invention, the conical portion of the shank extends directly to the drive portion or to the underside of the drive portion formed as the screw head. This ensures that the borehole will be steadily widened until the screw is finally set. Thereby there is also achieved a steadily increasing torque, which can be sensed by a driving tool with an adjustable stopping reaction.
In this regard it is also important to note that torque-activated stopping must take place with great precision, since the engagement of the thread in the substructure can be damaged under certain circumstances, thus negating the fastening effect, if driving is stopped too late.
It can also be advantageous if the diameter of the end of the conical portion of the shank close to the drive portion is larger than the outside diameter of the threaded portion on the shank. Since the screw is provided with a boring portion and thus itself creates the through-hole in the section to be fastened and also the hole in the substructure, naturally the threaded portion is also driven through the section to be fastened. Thus a corresponding thread is cut first of all in the borehole of the section or plate to be fastened, while the axial initial tensions will be developed by the conical portion in the threaded borehole of the section to be fastened. For practical purposes, therefore, the formed thread turns are compressed again or pressed flat by the conical portion, and thereby the friction between the conical portion and the borehole wall is smaller at least in the first two rotations than if the borehole wall had been smooth.
In the very cases in which the screw must be set at an exact depth, or in other words a drive portion formed as a screw head must always bear exactly on the surface of the section or plate to be fastened, the drive portion formed as the screw head can be additionally provided on its underside with projections and/or depressions. It is then possible to adjust the torque clutch of a driving tool to disengage not as soon as the conical portion exerts an effect, but only when an appropriately large torque increase is caused suddenly by the projections and/or depressions provided on the underside of the screw head, or in other words when the screw head makes full surface contact with the section or plate to be fastened. Because of the previous action of the conical portion, the section or plate to be fastened has already been pressed snugly against the substructure, and so all that remains is for the driving tool to be stopped by an appropriately massive torque increase.